This invention relates to a lithographic printing plate and a process for the preparation thereof. More particularly, it relates to a process for preparing a lithographic printing plate by utilizing a photosensitive silver halide emulsion.
The lithographic printing plate comprises lipophilic image areas receptive to a greasy ink and lipophobic non-image areas non-receptive to the ink, said non-image areas being generally made hydrophilic to receive water. The common lithographic printing, therefore, is performed by feeding both ink and water to the printing plate surface to allow the image areas to selectively receive the coloring ink and the non-image areas to selectively receive water, and transferring the ink on the image areas to a substrate material such as, for example, paper. In order to produce good prints, it is necessary that difference between the lipophilicity and the hydrophilicity of image areas and non-image areas be sufficiently large so that when an ink and water are fed to the printing plate surface, the image areas may receive a sufficient amount of the ink and the non-image areas may receive entirely no ink.
Photographic materials comprising a spectrally sensitizable silver halide emulsion of high sensitivity are suitable for the automatic printing plate making and are actually in use in several forms. Representatives of the methods for preparing lithographic printing plates by utilizing photosensitive silver halide emulsions are as follows:
(1) A method by utilizing the tanning development, in which the imagewise exposed hydrophilic gelatino-silver halide emulsion layer is subjected to tanning development to harden the gelatin of image areas, whereby the image areas are converted into a lipophilic ink-receptive pattern (U.S. Pat. No. 3,146,104).
(2) A method of preparing a printing plate, in which a metallic silver pattern formed on the exterior surface by the silver diffusion transfer technique is converted into a lipophilic ink-receptive pattern [U.S. Pat. Nos. 3,721,559 and 3,490,905; Japanese Patent Publication Nos. 16,725/73 and 30,562/73; Japanese Patent Application "Kokai" (Laid-open) No. 21,602/78; A. Rott and L. Dehoes, Journal of Photographic Science, 8, 26-32 (1960)].
(3) A method for preparing a lithographic printing plate by utilizing the etching bleach, in which the developed silver image areas or the areas of silver image formed by the transfer development are treated with a bleaching solution and, at the same time, the gelatin in the silver pattern areas is destroyed to expose the lipophilic surface (U.S. Pat. Nos. 3,385,701 and 3,814,603; Japanese Patent Publication No. 27,242/69).
(4) A method in which the undeveloped silver halide image areas in the developed hydrophilic gelatino-silver halide emulsion layer are selectively converted into a lipophilic ink-receptive pattern [U.S. Pat. Nos. 3,454,398, 3,764,323 and 3,099,209; Japanese Patent Application "Kokai" (Laid-open) No. 9,603/78].
As is well known, the matrix material used in the silver halide emulsion layer for lithographic printing plates is mostly gelatin. It is also known that an undercoat layer comprising gelatin as a hydrophilic polymer is provided on the support, as disclosed in Japanese Patent Application "Kokai" (Laid-open) Nos. 5,503/73, 100,203/73, 55,402/74, 21,602/78 and 9,603/78.
The gelatin-containing layer of the lithographic printing plate is generally hardened with a hardener such as formaldehyde to increase mechanical resistances of the printing plate for the purpose of ensuring a high printing endurance. The degree of hardening of the gelatin-containing layer is one of the important factors affecting the quality of lithographic printing plates. If the hardening is insufficient, the layer tends to be detached from the support during the printing operation, whereas if the layer is excessively hardened, there will occur scumming as well as blinding in the image areas, particularly where fine lines are involved, thus causing a reduction in the printing endurance. The causes for the occurrence of scumming and blinding in the latter case are yet to be elucidated, but seem to be originated in the reduced water retention or the change in development characteristics (such as characteristics of the transferred silver to be rendered ink-receptive in the diffusion transfer method for printing plate making). Therefore, there has been a strong desire among the concerned circles to develop a lithographic printing plate having a high printing endurance and free of scumming or blinding, which printing plate is capable of maintaining its degree of hardening always at a level optimum for the plate not only during its manufacture but also for a long period of time from its manufacture through its processing by the consumer into a printing plate.